This invention relates, generally, to downhole measuring instrumentation, and more specifically to flexible downhole instrumentation assemblies, such as may be used to negotiate short-radius angled wellbores. In one particularly preferred embodiment, the invention is in the form of a flexible downhole surveying instrument assembly for short-radius angle wellbores wherein the instrumentation components of the assembly are connected with a laterally flexible coupling.
In many applications, it would be desirable to have downhole instrumentation assemblies which will negotiate inclined wellbores, including short-radius bends, but which still have longitudinal rigidity. The need for such flexible instrumentation assemblies may encompass many types of instrumentation, including well logging apparatus and survey instrumentation. The need for this type of flexible instrumentation may perhaps be most severely felt with respect to well survey equipment used to determine the path of a well as it is drilled.
In both straight and directionally drilled holes, the position of the wellbore beneath the surface must be determined at various times and locations as the well is being drilled. This requires the use of surveying instruments that are able to measure the hole inclination and hole direction at various depths along the course of the well. The position of the wellbore relative to the surface location is then calculated from the cumulative survey results.
The surveying instrument records the direction and angle of the wellbore at a predetermined depth. To do this, the instrument is typically located near the bottom of the drill string in the wellbore.
Another use of the survey instrument is to measure and record the orientation of the drilling assembly relative to either magnetic north or to the high side of the wellbore. By holding the survey instrument in a desired position and taking surveys with the instrument at approximate intervals of 20 or 30 feet, for example, during the initial pilot bore (or first drill string used to guide larger drill pipe), the path of the short-radius bend in the wellbore may be calculated and plotted by inclination angle change and measured depth.
In one conventional type of survey assembly, a camera is incorporated into a surveying instrument assembly to record the wellbore inclination and direction. The camera photographs the inclination and direction measurements when the surveying instrument is in a stationary position near the bottom of the drill string. This type of downhole surveying instrument is known as the "single shot". Single shot surveys may be run at any depth. The interval is usually determined by the hole inclination build rate at the time the wellbore is being drilled. Magnetic multi-shot survey instruments are usually run at the completion of the wellbore prior to setting casing.
A single-shot or multi-shot downhole surveying assembly generally includes a sensitive angle measuring unit that indicates the magnetic direction and inclination of the wellbore (a compass and inclinometer), a camera unit with a lamp assembly to photograph the position of the angle measuring unit, and a timer or motion sensitive device that activates the camera. The surveying instrument assembly also may include a battery pack that provides the power required to operate the camera, timer and lights. The timer or motion sensing device is used to close an electrical circuit and activate the camera at the correct moment; for example, when the compass and inclinometer have reached the bottom of the wellbore. Typically, the timer can be set on the surface to allow sufficient time for the assembly to be run in the hole and landed in position before the camera operates. After the camera operates and the instrument is recovered at the surface, the survey results may be read directly off the developed film.
Multi-shot survey instruments are used to measure the trajectory of the wellbore. A multi-shot instrument is capable of taking a series of photographs at pre-set time intervals (e.g., every 30 seconds). Solid state sensors also have been employed with the multi-shot to measure the position of the wellbore. This type of instrument is often referred to as an electronic multi-shot. The results are stored in the tool's memory. Once the tool is recovered at the surface, the surveys can be obtained by linking the tool to a surface computer.
More recently, measurement systems which continuously monitor the inclination and direction of the wellbore have been used, which reduces the downtime required to run single-shot or multi-shot surveys. Continuous monitoring also gives the directional driller the ability to assess the effects of changing drilling parameters on borehole inclination, azimuth, and bottom hole assembly orientation. To survey the borehole position continuously while drilling typically requires the use of instruments that contain accelerometers and magnetometers. Accelerometers measure components of the earth's gravitational field, while magnetometers measure components of its magnetic field. By measuring the vector components of earth gravitational and magnetic field, the inclination and azimuth of the wellbore can be determined.
With each of these types of surveying instrumentation, it is fundamental to the survey operation that the instrumentation negotiate through or along the well path. Generally, the surveying instruments are contained in a cylindrical protective housing, which slides down either the wellbore or inside the drillstring assembly.
Short radius drilling typically establishes wellbores with increasing inclinations at rates of between 1.5.degree. to 3.degree. per foot and produces wellbore angle radii of between 20 and 40 feet. For example, the geometry of a well can change from vertical to horizontal in as little as 30 to 60 drilled feet. Short radius wellbores are often drilled with a drilling assembly consisting of a drill bit, rotating bit sub, a non-rotating curved drill guide, a rotating clutch sub and a flexible rotating drill pipe above. A flexible rotating drive shaft internal to the curved drill guide transmits the rotating torque from the rotating clutch sub to the rotating bit sub. An alternate drilling assembly may consist of a drill bit and mud motor with a bent housing.
Often, problems arise when attempting to lower or raise a surveying instrument assembly through this short-radius drill string assembly. Sinker bars often are used to add weight and help the surveying instrument travel through the mud and through the short radius to reach the landing plate at the bottom of the drill string assembly. However, even with sinker bars there is a risk of the instrument assembly sticking or hanging up at a short-radius curve in the drill string. Sinker bars generally are threadably coupled to one another, and to one or more spacer bars. Spacer bars are used to position the compass and camera assembly at the optimum point within the wellbore, typically to reduce the risk of magnetic interference.
Ideally, the surveying instrument housing, sinker bars, and spacer bars are slidable within the wellbore However, in wells with short-radius bends in the wellbore these instruments frequently stick and hang up at the outer cylindrical surface at various locations in the drill string. The cylindrical housing or canister for the surveying instrument, as well as the cylindrical sinker bars and spacer bars, while having enough lateral flexibility to pass a medium or long-radius angle wellbore, are typically too stiff and too large to traverse short-radius wellbores. Because of the cross-section or diameter of the housing, conventional housings simply are too stiff and fit too snugly within the bore of the drill string section that includes the drill guide and flexible joints to allow the instrument assembly to traverse the drill string. The present invention solves these needs and problems.